The formation of precise neuronal circuits during development is a highly regulated and dynamic process. Excess numbers of synapses are first generated to establish the initial wiring pattern of the brain, but the formation of mature, precise neuronal circuits requires the selective elimination and pruning of specific synapses. Neuronal activity plays a critical role in this refinement phase, but surprisingly, the specific molecular mechanisms underlying synapse elimination remain a mystery. In the adult brain, synapse loss often occurs long before the pathology and clinical symptoms in many neurodegenerative diseases. Identification of the instructive molecule(s) that mark synapses for elimination during development could also provide important clinical insight about therapeutic targets for devastating diseases such as Alzheimer's.
Synapses are specialized cell adhesions that are the fundamental functional units of the nervous system, and they are generated during development with amazing precision and fidelity. During synaptogenesis, synapses form, mature, and stabilize and are also eliminated by a process that requires intimate communication between pre- and postsynaptic partners. In addition, there may be environmental determinants that help to control the timing, location, and number of synapses.
Synapses occur between neuron and neuron and, in the periphery, between neuron and effector cell, e.g. muscle. Functional contact between two neurons may occur between axon and cell body, axon and dendrite, cell body and cell body, or dendrite and dendrite. It is this functional contact that allows neurotransmission. Many neurologic and psychiatric diseases are caused by pathologic overactivity or underactivity of neurotransmission; and many drugs can modify neurotransmission, for examples hallucinogens and antipsychotic drugs.
Glial cells associated with synapses, either astrocytes in the CNS or Schwann cells in the PNS, are thought to provide synaptic insulation by preventing neurotransmitter spillover to neighboring synapses and they also help to terminate neurotransmitter action. In addition, glial cells supply synapses with energetic substrates. The possible requirement of glia for synapse formation is suggested by the temporal association of synaptic development with glial development: although most neurons are born prior to the birth of most glia, the vast majority of synapses develop during the first few weeks of postnatal life, during the period of glial generation. For example, axons of retinal ganglion cells (RGCs) reach their target in the superior colliculus by embryonic day 16, but they do not form many synapses until the second postnatal week, coinciding with glial generation. Therefore the formation of many or most synapses is delayed until glial cells are present.
During development, competition between axons causes permanent removal of synaptic connections. The synapses to be eliminated become progressively weaker, are eliminated, and then the competing axon extends axonal processes to occupy those sites. These findings have lead to a simple model in which synaptic transmission produces two postsynaptic signals: a short range protective signal and a longer range elimination (punishment) signal. Functionally weak synapses are not protected from the elimination signal of neighboring stronger synapses, resulting in the disappearance of postsynaptic receptors and withdrawal of the axon. This withdrawal then provides the opportunity for the stronger axon to expand into the vacated territory. The identity of the punishment and protection signals have heretofore been unknown.
Shortly after birth, neonatal brains undergo a period of intense synaptic proliferation to levels far greater than those seen in adult brains. Later in infancy there is a spontaneous, normal period of synaptic pruning or reduction. In rhesus monkeys the synaptic density (i.e., the number of synapses per unit of brain tissue volume) peaks at 2 to 4 months of age and then gradually declines until about age 3 years, where it remains at adult levels. The proliferation and pruning appear to occur uniformly throughout the rhesus cortex.
Data on human brains suggest that these programmed fluctuations in synaptic density also occur, but they vary by brain region. Synaptogenesis in the visual cortex, for example, begins its rapid growth at about age 2 months, peaks at 8 to 10 months, and then declines gradually until about age 10 years. By contrast, synaptogenesis in the frontal cortex begins and peaks later, and pruning is not complete until adolescence. Interpretation of these findings about synaptic density counts is further complicated because synaptogenesis and pruning may occur at different rates in different structures within the same brain region or even for a particular group of neurons in different parts of their dendritic fields.
Two phenomena thought to be related to this process of synaptogenesis and pruning are those of so-called “critical periods” and neural plasticity, both of which have been studied extensively over the past 30 years. Deprivation of adequate sensory or motor input during particular times in a specific brain system's development (i.e., the critical period) can lead to impairments in that system's functioning, both at that time and in the future.
It is now thought that the need for appropriate sensory input is greatest during a brain system's period of rapid synaptogenesis and that experiential input helps shape the particular synaptic connections that are formed and also which ones are eliminated. This process corresponds to the “experience-expectant” type of neural plasticity that is tied to the brain's developmental timetable. By contrast, “experience-dependent” plasticity allows incorporation of useful but idiosyncratic information throughout life. The onset of critical periods and their durations vary widely over the different neural systems in the brain. At present, it is not known whether there are critical periods during which particular types of stimulation are needed and after which plasticity is greatly reduced. Nor is it known whether neuronal plasticity responsiveness is present in discreet, sensitive periods versus demonstrating more gradual decrease over time.
Although there are varied etiologies among neurodegenerative diseases, one cellular commonality which exists among all neurons is the synapse. Degeneration of functional synapses is of crucial importance to understanding the primary mechanisms of overall neurodegeneration. Evidence suggests that synapse loss precedes neuron loss, e.g. in early Alzheimer's Disease (AD). Several studies have correlated synapse loss with clinically defined neurological impairment. For example, statistical analysis has shown that synapse loss is more closely correlated with cognitive impairment in AD than are plaques and tangles. Moreover, a variety of proteins found in pathological hallmarks of neurodegenerative diseases are synaptic proteins or cleavage products of synaptic proteins. These include APP, amyloid precursor protein, alpha-synuclein, the precursor of NAC peptide found in Lewy bodies in Parkinson's Disease, and PrP.
These observations emphasize that synapse loss is a central event in neurodegeneration and that synaptic proteins have been involved in the neuropathology of disease. Despite this fundamental understanding, there has been little systematic study of synapse loss or the role of synaptic proteins associated with pathology. The modulation of synapse maintenance and loss is of great interest for the treatment of a variety of nervous system disorders. The present invention addresses this issue.